Attenuation correction in positron emission tomography (PET) is usually achieved using a transmission scan, which is typically performed with one or several rotating rod sources containing a positron emitter such as Ge-68. Since good statistics is required in the transmission scan, the imaging time is typically 10 to 20 minutes and the activity in the rod sources is as high as the scanner countrate capability permits. We propose to perform the transmission scan with singles gamma-ray emitter with an energy which is somewhat higher than 511 keV so that energy discrimination can be used to separate its energy from that of a positron emitter. This transmission source is shielded towards the nearer detector, allowing higher levels of activity to be used with the aim of reducing the total transmission time while still obtaining adequate statistical accuracy.